1. Field of the Invention
The present invention relates to an induction hardened hollow driving shaft suitable for reducing weight of a driving shaft that transmits an engine diving force of an automobile to respective wheels such as a drive shaft, in more detail, an induction hardened hollow driving shaft excellent in cold workability, hardenability, toughness and torsional fatigue strength that are demanded as fundamental characteristics of the driving shaft.
2. Description of the Related Art
Of automobile parts, in a drive shaft that is used as a driving shaft that transmits an engine driving force to wheels, as an automobile engine becomes higher in its output, a demand for higher strength is stronger. Normally, as the strength characteristics necessary for the drive shaft, the torsional fatigue strength is cited. Accordingly, hitherto, with a drive shaft having a solid structure, a drive shaft that exerts excellent torsional fatigue strength characteristics and steel that is used therefor have been proposed.
Although the deeper a depth of a hardened layer is, the more the fatigue strength of the driving shaft is improved, when the hardened layer is made excessively deep, there is a risk of quench crack. Accordingly, in Japanese Patent Application Publication No. 2000-154819, proposed is a high strength drive shaft where, in order to obtain a high strength drive shaft, an upper limit of a depth of the hardened layer is defined, and, in order to secure the hardness of the hardened layer, higher C and lower Cr are aimed in a composition design.
In the torsional fatigue fracture of an induction hardened member, since a crack occurs in a plane in parallel with a longitudinal direction of the shaft and at a boundary between a surface or a hardened layer and a core portion and initially propagates in a plane in parallel with the longitudinal direction, when elongated MnS is present in the longitudinal direction, along the elongated MnS, an occurrence and the initial propagation of the crack are promoted. Accordingly, Japanese Patent Application Publication No. 2002-69566 has proposed induction hardening steel where MnS is granulated and made finer. Thereby, the crack is inhibited from occurring and initiating the propagation and thereby the torsional fatigue strength can be improved.
The high strength drive shaft and the induction hardening steel proposed in the Japanese Patent Application Publication Nos. 2000-154819 and 2002-69566 are applied as a method of improving the torsional fatigue strength of a driving shaft that postulates a solid structure and expected to exhibit predetermined strength characteristics.
However, from a viewpoint of a recent further global environmental protection, it is strongly demanded to make an automobile body lighter to improve the fuel efficiency. Accordingly, there has been variously attempted to replace, in automobile parts, solid members with hollow members. As one of the attempts, as a driving shaft, a hollow structure is under study to adopt. When an automobile part is made a hollow structure, not only simple lightweight, but also an improvement in acceleration response owing to an improvement in the torsional stiffness and an improvement in the quietness in a running car owing to an improvement in the vibration characteristics can be expected.
In order to achieve such expectations, a development of a hollow driving shaft fabricated into a special shape is strongly demanded. For instance, in a design of a shaft that engages both shaft end portions with constant-velocity joints, while an intermediate portion of a driving shaft is made as small as possible in the thickness and as large as possible in the diameter to heighten the torsional stiffness and improve the vibration characteristics, the both shaft end portions that engage with the constant-velocity joints are made with a diameter same as that of the solid member used thus far, and thereby an existing constant-velocity joint can be favorably used as it is.
As a manufacturing method of a hollow driving shaft, there is a method where to both end portions of a hollow blank tube hollow or solid shafts are engaged by use of a friction welding method. However, in the method, it is difficult to make a diameter of the hollow portion larger and that of the both end portions smaller. From the above reason, in order to form a driving shaft having a shape where an intermediate portion is made as small in the thickness and as large in the diameter as possible and diameters of the both end portions are made small, a steel pipe material is cold worked to make the intermediate portion thinner in the thickness, followed by applying cold reducing process etc. to both ends of the steel pipe material to reduce outer diameters of the both end portions to thicken the wall thickness thereof, and thereby an integrally formed hollow driving shaft is manufactured.
In the integrally formed hollow driving shaft, in order to secure a special shape thereof, a complicated cold working is applied to form. Accordingly, in order to do so without the occurrence of the crack caused during the cold working and to secure the torsional fatigue strength after the working, as a raw material of an integrally formed hollow driving shaft, for instance, a seamless steel pipe is necessarily adopted.
When an integrally formed hollow driving shaft is manufactured with a steel pipe as a hollow shaft material, it is important to inhibit the crack due to the reducing process or spinning process at the pipe end from occurring. Furthermore, it is demanded that after the cold working, the heat treatment is applied to harden over an entire thickness of the steel pipe from an outer surface to an inner surface to secure high toughness and furthermore to secure the torsional fatigue strength so as to enable to obtain long lifetime as a product.
In other words, in the hollow driving shaft of which raw material is a steel pipe, it is indispensable to satisfy the cold workability that enables to easily obtain a complicated shape, the hardenability accompanying the heat treatment, and the toughness and torsional fatigue strength, and to achieve stable fatigue lifetime as the driving shaft. However, in the hollow driving shafts hitherto proposed, studies from the viewpoints of materials or the grain boundary strength have hardly done.
For instance, in Japanese Patent Application Publication No. 6-341422, a drive shaft where a balance weight is attached to a driving shaft steel pipe to reduce the rotational whirling is disclosed, and it is further disclosed that, when values of carbon equivalents (Ceq=C+Si/24+Mn/6+Cr/5+Mo/4+Ni/40+V/14) of the driving shaft steel pipe and the balance weight are defined, the fatigue fracture generated from a site where the balance weight is welded can be reduced.
However, when only values of carbon equivalents (Ceq) of the driving shaft steel pipe and the balance weight are defined, the driving shaft steel pipe excellent in both the cold workability and the fatigue characteristics cannot be obtained. Accordingly, it is difficult to apply an automobile drive shaft disclosed in the Japanese Patent Application Publication No. 2000-154819 as an integrally formed hollow driving shaft.
In the next place, in Japanese Patent Application Publication No. 7-18330, a manufacturing method of high strength and high toughness steel pipe suitable for a high strength member used in automobile underbody members is proposed. In the manufacturing method according to the disclosure, a specific composition system is defined. However, since neither Ti is added nor N is defined, even when B is added, it is not a composition system that can secure sufficient hardenability. Furthermore, since neither the cold workability nor the fatigue characteristics is considered in the composition design, in the manufacturing method proposed in Japanese Patent Application Publication No. 7-18330, it is difficult to obtain an integrally formed hollow driving shaft.
On the other hand, Japanese Patent Application Publication No. 2000-204432 discloses a drive shaft where the induction hardening is applied to graphite steel to harden a superficial layer and to generate two-phase structure of ferrite and martensite at a core portion. However, a chemical composition that the Japanese Patent Application Publication No. 2000-204432 discloses shows a composition system suitable for a hollow driving shaft steel material for use in the friction welding and it takes a long heat treatment time to obtain graphitized steel. Furthermore, since the chemical composition does not include Cr and the hardenability and the fatigue strength are insufficient, an integrally formed driving shaft cannot be obtained.
In Japanese Patent Application Publication No. 2001-355047, as a raw material of a drive shaft, a high carbon steel pipe that has a grain diameter of cementite of 1 μm or less and is excellent in the cold workability and the induction hardenability is proposed. However, in the high carbon steel pipe proposed in Japanese Patent Application Publication No. 2001-355047, since the warm working is necessary to obtain a target microstructure, the manufacturing cost goes up. At the same time, the disclosed steel composition cannot constitute an integrally formed hollow driving shaft simultaneously satisfying the cold workability, the hardenability and the fatigue characteristics.
In order to not only simply achieve lightweight but also achieve an improvement in acceleration response owing to an improvement in the torsional stiffness and in the quietness in a running car room owing to an improvement in the vibration characteristics, a development of hollow driving shaft is necessary. When a solid driving shaft is manufactured, as the heat treatment, the surface hardening is applied. On the other hand, when a hollow driving shaft is manufactured, in order to secure sufficient strength, it is necessary to apply the hardening over an entire thickness to an inner surface of the driving shaft.
As described in Japanese Patent Application Publication No. 2002-69566, in the torsional fatigue fracture in the solid diving shaft, a crack is generated in a plane in parallel with the longitudinal direction at a boundary between a surface or a hardened layer and a core portion. On the contrary, according a study by the present inventors, the torsional fatigue fracture in the hollow diving shaft is generated in a direction that is at 45° to the longitudinal direction, and in a principal stress plane. This is because while in the solid driving shaft a deformation energy that accompanies load of the torsional torque is absorbed by a low hardness region inside of the solid driving shaft, in the hollow driving shaft such an action of absorption of the deformation energy is not generated.
According to a further study of the present inventors, in the hollow driving shaft, owing to the load of the torsional torque, the intergranular fracture tends to occur. In particular, when the intergranular fracture occurs at an early stage, the torsional fatigue fracture rapidly progresses and the fatigue lifetime of the driving shaft becomes obviously instable. The instability of the fatigue lifetime as well is assumingly caused by the fact that in the hollow driving shaft the deformation energy accompanying the torsional torque is not absorbed in a low hardness region inside of the shaft.
Thus, in the hollow driving shaft and the solid driving shaft, owing to difference of the hardened microstructures due to the heat treatment, fracture behaviors under torsional torque load are different. Accordingly, in order to improve the torsional fatigue fracture of the hollow driving shaft and to stabilize the fatigue lifetime thereof, improvement methods of the torsional fatigue strength proposed in Japanese Patent Application Publication Nos. 2000-154819 and 2002-69566 cannot be applied. That is, in the hollow driving shaft, since owing to the load of the torsional torque the intergranular fracture tends to occur, in order to improve the torsional fatigue fracture of the hollow driving shaft and to stabilize the fatigue lifetime thereof, the strength of a prior austenite grain boundary is necessarily secured.
On the other hand, when a steel pipe is used as a raw material of the hollow driving shaft, it is necessary that the crack due to the reducing process or spinning process at the pipe end is inhibited from occurring, the heat treatment is applied after the cold working to harden an entire thickness through an inner surface of the steel pipe and to secure high toughness, and furthermore in order to display excellent performance as the hollow driving shaft, the cold workability, the hardenability, the toughness and the torsional fatigue strength are simultaneously secured.
However, in proposals of Japanese Patent Application Publication Nos. 6-341422, 7-18330, 2000-204432 and 2001-355047, there is hardly found an attempt where so as to exhibit, as a hollow driving shaft with a steel pipe as a raw material, excellent cold workability, the hardenability, the toughness and the torsional fatigue strength, raw materials and grain boundary strength are studied and thereby the chemical composition and grain diameter are specified.
In other words, each of the characteristics that the hollow driving shaft demands is not difficult to improve individually. However, according to existing knowledge, it is considered difficult to simultaneously satisfy all the characteristics. For instance, since, in order to secure high fatigue strength, the strength of the steel is effectively increased, when the steel pipe that is used as a raw material is made high in the strength, whereby the cold workability is deteriorated accordingly.